Mechanosensing is currently rediscovered as a major process controlling plant growth and development. External mechanical loadings lead to a decrease in elongation and a stimulation of cambial growth, a syndrome known as thigmomorphogenesis. In their environment, plants are submitted to successive loadings so that most of studies of thigmomorphogenesis consist in applying successive loadings and to measure their global effect on plant growth. But as plants seem to acclimate when regularly loaded, studying plant responses after a single loading appear as a prerequisite for understanding and characterizing the regulation of plant mechanosensing under several mechanical loadings. Moreover, previous work has demonstrated that plants perceive the strains they are subjected to and not forces or stresses. From this fact, a biophysical model of mechanosensing was established in the case of tomato elongation but its underlying assumptions remained to be tested using local responses like cambial growth or gene expression.
In a first time, we characterized the poplar cambial growth response after a single bending. An original bending device was conceived enabling to impose a controlled level of strains. Growth response was studied according to a range of imposed strains. A single transitory bending was sufficient to modify the plant cambial growth for several days. The cambial growth response was highly correlated with the sum of longitudinal strains revealing that the biophysical model of mechanosensing established for tomato stem elongation held for cambial growth.
In a second time, we aimed at testing of the underlying assumptions of the model of mechanosensing at molecular scale. For doing so we looked for a mechanosensing marker gene: a mechanosensitive gene that would express rapidly and only in the strained zone. In poplar, we recently identified a primary mechanosensitive gene, named PtaZFP2. This gene is a member of the Cys2His2 zinc finger protein family. We studied its level of expression after a single bending according to a range of strains. Interestingly, integrals of longitudinal strains induced by bending were highly correlated to PtaZFP2 mRNA relative abundance. The model of mechanosensing established for stem elongation thus also held at the molecular level. To our knowledge it is the first time a quantitative link is established between the applied level of strains and cambial growth as well as the level of expression of a mechanosensitive gene.
Taking together, these results open new areas concerning system biology of plant under mechanical loading and underline the interest of pluridisciplinary research.
This work was a prerequisite for analyses of plant acclimation under several mechanical loadings. This work is currently under progress.